1. Field of the Invention
The present invention relates generally to a method and apparatus for improving the quality of an image, and more particularly to removing uneven brightness in an image from a particle based image system.
2. Related Art
A potential problem with imaging using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and other particle illumination based imaging systems, is the difference in brightness visible in regions of a surface of some materials. While this brightness difference may provide useful information by denoting the boundaries of uneven surfaces or structures, there are applications in which this uneven brightness can be problematic. These problematic regions may also include regions in which the brightness level is too high, making boundaries and other structures difficult to identify.
In an electron microscope or FIB, charged particles are accelerated and focused in a beam at a sample to be inspected. The interactions of the particles with the sample create a number of different emissions such as forward-scattered (transmitted) electrons, back-scattered electrons, secondary electrons, Auger electrons and X-rays. The brightness level in most images created from the detection of these emissions is related to the quantity of detected emissions.
Brightness artifacts occur when detection of emissions is affected by interference from nearby interior or surface structures of the sample.
In an SEM image of an integrated circuit, the brightness difference can generally be seen as shadowing and is most noticeable around interconnects and the edges of metal structures where surfaces are uneven. It is particularly noticeable in banks of interconnects. This shadowing makes automatic segmentation and manual study of the images difficult.
Some prior art methods include techniques for correcting contrast by comparing two images of the target object and correcting the target of interest based on the differential information.
U.S. Pat. No. 6,587,581, issued to Matsuyama et al on Jul. 1, 2003, discloses a method of correcting the difference in brightness between comparison images taken of a target area of interest, and a reference area having a corresponding pattern. This method has the disadvantage of requiring a reference area elsewhere in the image with a repeating pattern. In images in which there are no repeating patterns, this method would not be effective.
U.S. Pat. No. 5,715,334, which issued to Peters on Feb. 3, 1998, discloses a method to “smooth” contrast for detail enhancement of an image. Two smoothed images of the same target, each processed with a different smoothing factor, are compared to obtain differential information between least smoothed and most smoothed data sets. This method has the disadvantage of requiring multiple smoothing techniques and methods of comparing the two different images. Further, smoothing techniques tend to blur the image.
Other well-known brightness correction methods include techniques using histogram stretching or hysteresis filtering. These methods however, generally apply to brightness correction across an entire image and not to regions within an image.
Therefore, there is a need for a simpler technique for correcting the uneven brightness effects in an image.